Process for the coating of electrodes

ABSTRACT

ELECTRODES COATED WITH RUTHENIUM DIOXIDE ARE MANUFACTURED BY APPLYING AN ANCHORING LAYER CONTAINING AT LEAST ONE COMPOUND OXIDZABLE BY RUTHENIUM TETROXIDE TO THE AREA OF AN ETCHED METAL SUPPORT WHEREIN THE RUTHENIUM DIOXIDE IS TO BE FIXED, EXPOSING THE THUS COATED SUPPORT TO RUTHENIUM TETROXIDE IN THE GASEOUS STATE, WHICH IS DECOMPOSED TO RUTHENIUM DIOXIDE UPON CONTACT WITH THE ANCHORING LAYER ON WHICH IT IS PREFERENTIALLY FIXED AND THEN HEATING THE THUS TREATED SUPPORT. ELECTRODES PRODUCED IN THIS MANNER HAVE AN ADHERENT COATING OF RUTHENIUM DIOXIDE WHICH IS RESISTANT TO ELECTROLYTE CORROSION AND SUPPORT HIGH CURRENT DENSITIES.

United States Patent 3,668,005 PROCESS FOR THE COATING OF ELECTRODES Guy Sluse, Rixensart, and Gustave Joannes, Abolens, Belgium, assignors to Solvay & Cie, Brussels, Belgium N0 Drawing. Filed Jan. 11, 1971, Ser. No. 105,628 Claims priority, applicatgnlguxemburg, Jan. 9, 1970,

rm. (:1. new N18 US. Cl. 117-215 13 Claims ABSTRACT OF THE DISCLOSURE Electrodes coated with ruthenium dioxide are manufactured by applying an anchoring layer containing at least one compound oxidizable by ruthenium tetroxide to the area of an etched metal support wherein the ruthenium dioxide is to be fixed, exposing the thus coated support to ruthenium tetroxide in the gaseous state, which is decomposed to ruthenium dioxide upon contact with the anchoring layer on which it is preferentially fixed and then heating the thus treated support. Electrodes produced in this manner have an adherent coating of ruthenium dioxide which is resistant to electrolyte corrosion and support high current densities.

BACKGROUND OF THE INVENTION The present invention pertains to electrodes used in electrochemical processes composed of a metallic support which conducts electricity and which is resistant to corrosion under the conditions prevailing in an electrochemical cell and a. metal oxide coating fixed on the support which is also resistant to electrochemical corrosion and which favors the exchange of electrons between the support and the ions of the electrolyte. More particularly, this invention concerns a process for manufacturing a metal sup port coated with ruthenium dioxide.

Recently, various types of metal electrodes have been developed having a coating which is comprised of at least one metal oxide of the platinum group. It has been observed that with usage, there is a relatively rapid deterioration of the electrochemical characteristics of these electrodes, which causes, in addition to the necessity of their replacement, the contamination of the products of the electrolysis and a decrease in the efficiency of the electric current.

Various methods have already been proposed for the production of metal oxide coatings of the platinum group, which are obtained either directly as oxide or in the metallic state; in the latter case, the metal coating is converted to the oxide by baking in an oxidizing atmosphere, by heating at high frequency under vacuum, by electrolysis in pulsated current or by immersion in a molten bath of an oxygenated salt.

In most of the proposed coating methods, the coating is carried out in the liquid phase, generally by applying a solution or suspension of a compound of the platinum group to the metal support by repeated painting by immersion or by spraying, after which the oxide is precipitated by chemical, thermal or electrical means. Oxides of the platinum group of metals can also be deposited directly on the metal support starting with these solutions or suspensions by electrolysis with an alternating current, or by electrophoresis, and also by immersion of the Patented June 6, 1972 metal support in a molten bath of an oxide of a metal of the platinum group under pressurized oxygen. Finally, coating methods have also been described which are carried out by electrostatic pulverization under vacuum or in the presence of oxygen and even by means of a plasma generator.

SUMMARY OF THE INVENTION One of the objects of the present invention is to remedy difliculties encountered with known metal electrodes coated with a metal oxide of the platinum group by providing a low over-voltage stable electrode which is especially suitable as an anode for the electrolysis of aqueous solutions of alkali metal halides, and which also may be used advantageously for other electrolytic and electrochemical techniques, such as the production of peroxide salts, for the protection of the cathode, for the oxidation of organic compounds and for fuel batteries.

Another object of the present invention is to furnish a simple and relatively inexpensive process for the production of such an electrode.

A process for producing a coated electrode has been discovered which does not require the installation of costly apparatus and which may be carried out very economically, particularly with respect to the labor required, compared with that of the processes commonly employed at present, especially those coating processes which involve the application of successive coats of material. Further, by means of the present invention, electrodes are obtained with an adherent coating which is completely resistant to electrolytic corrosion, particularly to nascent chlorine, and which develops with the release of chlorine a very low over voltage which hardly varies with time and, in addition, the electrodes support high current densities.

According to the present invention, the coating process is carried out by applying an anchoring layer containing at least one compound oxidizable with ruthenium tetroxide to those areas of a previously etched metal support wherein ruthenium dioxide is to be fixed, exposing the thus coated support to an atmosphere containing ruthenium tetroxide in the gaseous state, the ruthenium tetroxide being decomposed to ruthenium dioxide on contact with the anchoring layer on which it is preferentially fixed, and then heating the thus treated and coated support.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The support is generally composed of a film-forming metal, such as titanium, tantalum, zirconium, niobium and tungsten, or of an alloy consisting principally of at least one of the foregoing metals, i.e. containing at least 50% by weight of one or more of the foregoing metals.

The etching of the support is effected by any known means such as electrolysis, immersion in a molten bath of alkali salts or their mixtures, or in an aqueous solution of alkali or organic or inorganic acid; but, generally, immersion in an aqueous solution of oxalic acid or hydrochloric acid is preferred.

The composition applied to form the anchoring or adhesive type layer must have a syrupy, i.e. slightly to moderately viscous, liquid consistency so as to form a continuous layer, i.e. a coating without gaps, on the surface of the metallic support which is to be coated eventually with ruthenium dioxide. The adhesive may be composed of any compound or mixture thereof which is oxidized by ruthenium tetroxide. Ruthenium tetroxide is a strong oxidizing agent and is known to oxidize a wide variety of organic and inorganic compounds, as indicated by numerous publications, for example, Berkowitz et al., J. Am. Chem. Soc. 80, 6682 (1958); Benyon et al., Proc. Chem. Soc. 1964 (Oct.), 342; Caputo et al. Tetrahedron Lett.l967 (47), 4729; Benyon et al., Carbohyd. Res. 1968, 6(4), 431; Rylander, Engelhard Ind. Tech. Bull. 1969, 9(4), 135 and US. Pat. No. 3,278,558.

Thus, the anchoring layer may contain any compound or mixture thereof which is oxidized by ruthenium tetroxide such as aromatic compounds, olefins, alcohols, aldehydes, amides, ethers, sulphides, hydrides, plasticizers for polymers, oils such as coriander oil, paraflin oil, silicone oil, greases such as silicone grease, hydrocarbon greases, or mixtures of these substances. The oxidizability of a variety of specific compounds in the aforementioned classes by means of ruthenium tetroxide is readily available in the literature, several examples of which have already been specified.

In order to facilitate their application onto the support, when desirable, any of the foregoing substances may be placed in solution or in dispersion in a solvent such as titanium tetrachloride, carbon tetrachloride, trichloroethylene, perchloroethylene, methylene chloride, benzene, toluene, petroleum ether, gasoline or petroleum. The main role of the solvent is to reduce the viscosity of the coating in orderto provide a homogeneous and continuous coating of the support. The solvent may likewise play a role with respect to the reduction of gaseous ruthenium tetroxide, The coating can be carried out by any adequate technique, such as painting, smearing, spraying or immersion.

Due to their commercial availability, silicone and parafin oil and grease are preferably used for the anchoring layer of the present process. Among other compositions, oils and greases, such as silicones, for example polydimethylsiloxane and paraflins, for example Vaseline, which are in general use as lubricants for valves, taps, bearings and so forth provide the desired adhesive and oxidative properties required of the anchoring coating ofthe invention. Such compositions are sufficiently viscous to prowide continuous coatings on the metallic supports, which coatings are capable of catching and anchoring the ruthenium dioxide thereon.

The exposure of the coated support to ruthenium tetroxide can be carried out in the presence of air as well as in an inert atmosphere. The temperature and the pressure must be such that the ruthenium tetroxide is in the gaseous state and the operation is generally carried out between 20 and 300 C. under a pressure close to the atmospheric pressure. The period of exposure to gaseous ruthenium tetroxide may varyfrom 10 minutes to 10 hours depending on the particular conditions, i.e. temperature, and the nature of the anchoring layer.

Preferably, the exposure to ruthenium tetroxide is carried out at a temperature above 40 C. and in an atmosphere composed essentially of air, water vapor and RuO The thermal treatment is carried out in an atmosphere containing oxygen at a temperature of about 200 to 550 C. For convenience, the support which has been coated with an anchoring layer and then exposed to ruthenium tetroxide in the gaseous state is generally heated in the presence of air under a pressure equal or inferior to atmospheric pressure. Preferably, the periods of heat treatment does not exceed hours. The particular objects of the treatment with heat include the oxidation of the remaining anchoring layer and the crystallization of the deposited oxide.

Preferably, the anchoring layer, prior to treatment with gaseous RuO, is of a thickness of about 0.1 to 24 g./m. and more preferably about 0.5 to 12 g./m.'*. Although there is not a direct correspondence between the thickness or wt./unit area of the anchoring layer and amount of Ru0 in the final layer, an anchoring layer which is thinner than specified generally does not provide a coating of RuO of the preferred thickness and, when the anchoring layer is too thick, an excessive deposit of RuO is often obtained which causes a decrease in the electrical polarization of the electrode. The thickness of the final coating, after thermal treatment, is preferably about 0.1 to 6 g./m. and more preferably about 0.5 to 4 g./m. when obtained in a single series of operations.

The sequence of steps according to the invention, i.e. application of the anchoring layer, exposure to gaseous ruthenium tetroxide, may be repeated several times so as to obtain a coating of the desired thickness. When the sequence of steps is repeated, the final thermal treatment can be carried out driectly after the final application of the anchoring layer, i.e. the step of exposing the support provided with the anchoring layer to ruthenium tetroxide,

may be omitted from the last sequence of steps.

In a particularly preferred embodiment of the invention, the metal support material is titanium or one of its alloys having anodic polarization properties similar to those of titanium. Such an electrode is particularly suitable as the anode in the electrolysis of aqueous solutions of alkali metal halides.

The examples which follow further illustrate the best mode currently contemplated for carrying out the invention but must not be construed as restricting the invention in any manner.

EXAMPLE 1 Small plates of titanium which have been etched by immersion over a period of 5 hours at approximately 100 C. in an aqueous solution of 10% oxalic acid are coated with PB IV parafiin oil (Pharmacope Belge, IVme edition) with the aid of a cloth or impregnated wiping paper and then suspended in a sealed enclosure, over an acid solution of ruthenium sulfate obtained by heating a solution of ruthenium chloride in the presence of sulfuric acid until the chlorine ions have been eliminated completely from the solution.

A solution of potassium permanganate is added to the solution of ruthenium sulfate and then the temperature is slowly increased. The rapid formation of a black deposit of ruthenium dioxide is observed on the small plates of titanium in the areas coated with parafiin oil, resulting from the reduction of gaseous ruthenium tetroxide released by the reaction. The exposure to gaseous RuO is continued until such time that the temperature reaches C., which takes approximately 2 hours. The small plates are then subjected to a thermal treatment in the air for a period of 15 hours at different temperatures, as indicated in Table 1.

The electrodes produced as described above were tested as anodes in the electrolysis of a circulating brine saturated with sodium chloride and chlorine at 60 C., in the presence of a cathode of platinum and their polarization voltage was measured during the course of electrolysis in comparison with a saturated calomel electrode using a Luggin siphon. The results have been set forth in Table l. The active surface of each electrode is 1 cm.

EXAMPLE 2 The same small plates of titanium, etched as in Example l, were coated with SISS-SI silicone grease for valves (supplied by the Socit Industrielle des Silicones, Paris, France). This grease has the consistency of petroleum jelly at room temperature and retains its consistency from 40 to 500 The grease was applied by using different techniques, in dispersion and in solution in 'various solvents, as indicated in Tables 2, 3 and 4. After being coated with grease, the small plates were coated with ruthenium dioxide according to the technique set forth in Example 1. The thermal treatment was carried out in air for a period of 15 hours at a temperature of 400 to 450 C.

The electrodes thus obtained were tested as anodes under the same conditions as set forth in Example 1 and the results are set forth in Tables 2, 3 and 4. The active surfaces are 1 cm.

Five of these electrodes must be considered to be unsatisfactory since their electrical polarization is below 50 ma. under 1.800 v. The low electrical polarization of these electrodes must be attributed to an excessive thickness of the anchoring layer which, in the case of silicone grease, should never exceed 23 g./m. and preferably be under 15 g./m. An anchoring layer which is too thick often results in an excessive deposit of ruthenium oxide. The final coating weight should not exceed g./m. after the final thermal treatment during which the silicone grease is volatilized, and particularly that which has not reacted with gaseous RuO EXAMPLE 3 The same small plates of titanium, etched in the manner set forth in Example 1, were coated by means of immersion in solutions or emulsions having various concentrations of SISSSI silicone grease for valves in methylene chloride. After immersion in the coating baths which has a very high viscosity (emulsions having a high concentration and above all pure grease at 100% the small plates were subjected to wiping with paper in order to eliminate any possible discontinuities or gaps in the coating layer and in order to eliminate the excess coating in particular at the base of the small plates.

As in the preceding examples, the small plates were then suspended over the acid solution of ruthenium sulfate in a sealed enclosure, but the technique of exposure to the vapors of 'RuO was modified after adding the solution of potassium permanganate, the temperature was rapidly increased while placing the enclosure in an oven at 90 C. where it was kept for a period of 4 hours. Finally, the small plates were heated to a temperature of 400 to 450 C. in the air for a period of 15 hours.

The electrodes thus obtained, whose active surface was still 1 cm. were tested as anodes under the same conditions set forth in Example 1. The results are set forth in Table 5.

It is noted that the wiping reduces the thickness of the anchoring layer for the high concentration baths so that in no case the final coating weight exceeds 5 g./m. All of the electrodes in this example show extremely advantageous polarization characteristics which are maintained during the course of time.

EXAMPLE 4 In the manner set forth hereinabove, a small plate of titanium, identical to the plates described in the preceding examples, is etched and coated with an anchoring layer of 2.5 g./m. by means of immersion in undiluted M 1028 silicone oil (Union Chimique Belge), which has a viscosity of 50 centistokes, a flash point of 342 C. and a self-ignition temperature of 485 C. The coated plates are then treated with gaseous RuO, and subjected to a thermal treatment in accordance with the techniques described in Example 3. The final coating weight was 3.51 g./m.

Tested as an anode under the same conditions as those set forth in the preceding examples, the small plate thus coated showed a polarization of 950 ma. under a voltage of 1.800 v.

EXAMPLE 5 The same small plates of titanium which are etched in the same manner as set forth in Example 1 were coated by means of immersion in solutions of SISS-SI silicone grease or M 1028 silicone oil in titanium tetrachloride and then covered with ruthenium dioxide and subjected to thermal treatment in accordance with the technique described in Example 3.

The electrodes thus obtained, whose active surface is still 1 cm. were tested as anodes for the electrolysis of circulated brine saturated with sodium chloride and chlorine at C., in a cell having a moving mercury cathode and the variation of the electrolysis current was measured over a period of time for a constant voltage of 7.9 v. at the terminals of the cell. The results are set forth in Table 6.

It is noted that after 72 hours of electrolysis under a voltage of 7.9 v., a high polarization current is main tained.

EXAMPLE 6 After etching as indicated in Example 1, the small plates of titanium were coated by immersion in the silicone grease or oil already mentioned containing various quantities of pulverulent titanium hydride in suspension. After the elimination of the excess coating by wiping with paper, the small plates were then covered with ruthenium di oxide and subjected to a thermal treatment in accordance with the technique described in Example 3.

The small plates thus coated were tested as anodes in the same conditions as indicated in Example 5 and showed polarizations indicated in Table 7. Here too, the high polarizations are maintained over a period of time.

COMPARATIVE EXAMPLE A comparative test was carried out on titanium electrodes coated with RuO by means of the reduction of gaseous RuO on silicone grease according to the present invention, titanium electrodes coated with RuO obtained by painting a solution of ruthenium chloride on the electrodes and titanium electrodes coated with a mixture of RuO -TiO containing 31 mole percent of Ru0 likewise obtained by painting. These electrodes were tested as anodes for the electrolysis of a brine under the conditions described in Example 1, under anodic current densities of 4 ka./m. and 8 ka./m. The potentials as compared with the saturated calomel electrode were measured in motion, while the anodes effectively delivered a current of 4 or 8 ka./m. The results are given in Table 8 in mv.:

After 72 hours of electrolysis, the potential of the electrode in accordance with the present invention was maintained at 1650 mv. under 8 ka./m. with variations of 10 mv.

Under the same conditions, the potential of the Ru0 coated electrode, obtained by painting remained at approximately 1800 mv. for a period of 24 hours and then it increased considerably to 2250 mv. after 46 hours.

The coating process of the present invention has indisputable advantages over the known processes. Using the process of the invention, it is possible to treat a large number of electrodes simultaneously and the process is easily adapted to integral automation of the sequence of required steps. Further, and above all, the process of the invention provides electrodes having an adherent coating with advantageous and durable electrochemical characteristics, in particular with respect to anodes used for the electrolysis of aqueous solutions of alkali metal halides. Finally, the process of the invention leads to the formation of a deposit of isotropic oxide having maximum electric conductivity.

TABLE 3 Technique for applying anchoring layer: Brush Solvent for the anchoring composition. -.-.'.:.:.33.1.1: CHzClz CHzClg CHzClz CHzClz C014 C014 0014 02014 Concentration of the solution, percent 5 10 20 10 Anchoring layer (gJm. 6. 1 9. 9 26. 1 48. 2 4. 6 11. 8 21. 7 4. 6 Final coating lin 4.93 1.64 6. 07 17.1 1. 03 1. 26 1. 1.98 Polarization (mv./300 ma.) 2,100 1,600 2,000 2,000 1,710 1,820 1, 500 1,900 Polarization (ma./1.800 v.) 130 430 17 6 380 290 570 230 TABLE 4 Technique for applying anchoring layer: Immersion Solvent for anchoring layer CHzClz C014 C2014 Concentration of solution, percent 5 1O 20 30 5 10 20 10 Anchoring layer (gJmJ)--. 1. 4 4. 6 24. 8 146. 5 1. 1 6. 9 46. 0 4. 2 Final coating-.- 1. 07 l. 5. l8 1. 64 1. 29 1. 79 26. 3 1.37 Polarization (mv./300 ma.) 2,000 1,800 2,000 2,000 1,650 2, 000 2,000 1,360 Polarization (ma/1.800 v.) 160 390 5 0. 1 400 170 l. 4 770 TABLE 5 Technique of applying anchoring layer Immersion without wiping With wiping Percent of silicone grease in the coating bath 0. 1 1 1 3 3 5 5 5 10 100 Weight of the anchoring layer (gJmfi) O. 2 0. 5 0. 5 1. 6 1.4 4. 2 3. 2 0. 7 0. 5 0. 7 Weight of the final coating (gJmfi) 2. 10 0. 61 2. 82 4. 96 4. 50 2. 2. 43 4. 27 3. 62 3. 27 Polarization (mv./300 ma.) 1,370 1, 345 1, 345 1, 385 1,410 1,345 1, 320 1,380 Polarization (ma/1.800 v.) 780 900 925 950 10. 50 840 770 870 960 730 TABLE 6 5. Process according to claim 1, in which said metallic S a support consists essentially of titanium or an alloy thereof 1 Grease having anodic polarization properties substantially the o1 ionn;.; 1ay or (g./ 1 n. 4. 4% 3.08 3.35 30 same as that of titanium. ,i? g% j "'T:)' 5 39$ 6. Process according to claim 1, in which said com- Po ari at on after 72 hours -l -9 v-) 3- 3.3 pound is at least one member selected from the group consisting of an organosilicon compound and a parafiin TABLE 7 and in which said anchoring layer is applied to said sup- Silicone compound Grease Grease Oil 35 port m admlxture i l H d d t u t m i h 7. Process according to claim 1, m which said comy ri econ en percen we g t 50 10 60 Final coating (g./m. 3. 60 3. 3. 80 Pound ls paraffin Initial polarization (a./7.9 v.)- 3.6 4,1 as 8. Process according to claim 1, m which the sequence Pmamamm after 721m (3479 M 7 of steps including application of the anchoring layer, ex-

40 posure to gaseous ruthenium tetroxide and the thermal What we claim as new and desire to secure by Letters Patent is:

1. Process for manufacturing an electrode for electrochemical processes wherein the electrode is composed of a metallic support which conducts electric current and is resistant to corrosion under the conditions prevailing in the electrochemical cell, and a coating which is resistant to electrochemical corrosion consisting essentially of ruthenium dioxide fixed on at least part of the surface of the support which comprises applying an anchoring layer containing at least one compound oxidizable by ruthenium tetroxide on the area of said support where said coating is to be fixed, exposing said support provided with said anchoring layer to ruthenium tetroxide in the gaseous state whereupon the ruthenium tetroxide is preferentially fixed on the area of said support provided with said anchoring layer, and then subjecting the support to a thermal treatment.

2. Process according to claim 1, in which the exposure to said gaseous ruthenium tetroxide is carried out at a temperature of about 20 to 300 C. and under a pressure of about atmospheric pressure.

3. Process according to claim 1, in which the thermal treatment of the coated support is carried out in the presence of air at a temperature of about 200 to 550 C.

4. Process according to claim 1, in which said metallic support consists essentially of titanium, zirconium, niobium, tantalum, tungsten, or an alloy constituted principally of at least one of the foregoing metals.

treatment is repeated a plurality of times.

9. 'Process according to claim 8, in which in the final sequence of steps, after applying the anchoring layer to said support, the step of exposing the support to gaseous ruthenium tetroxide is omitted and the coated support is subjected directly to a thermal treatment.

10. Process according to claim 1, in which said compound is an organosilicon compound.

11. Process according to claim 10, in which the organosilicon compound is selected from the group consisting of a silicone oil and a silicone grease.

12. Process according to claim 10, in which the organosilicon compound is polydimethylsiloxane.

13. Electrode obtained by the process of claim 1.

5 References Cited UNITED STATES PATENTS 2/1971 Martinsons.

FOREIGN PATENTS 1,147,442 4/1969 Great Britain.

ALFRED L. LEAVI'IT, Primary Examiner 5 C. K. WEIFFENBACH, Assistant Examiner US. 01. x.R.

l17-106, 161 z A, 218, 230; 204-290 F 

